Deciphering swift reversal of multifaceted photodynamics of a novel pyrene appended unsymmetrical salicylaldehyde azine derivative in aqueous and protein environments

Abstract

In this work, we report a novel pyrene tethered unsymmetrical monosubstituted salicylaldehyde azine based organic functional molecule named 4-chloro-2-((E)-((E)-(pyren-1-ylmethylene)hydrazono)methyl)phenol (PHCS) and we performed comprehensive photophysical investigations on variable states of the fluorophore through extensive experimental and theoretical techniques. This luminophore possesses diverse multifaceted photophysics in different microenvironments, resulting in altered multifluorescence emission behaviors of the same molecule in different states. This state-dependent photophysical behavior of PHCS is attributed to the combinatorial (or tuning) effect of different photophysical mechanisms. The solution photophysics is dominated solely by the photo-induced electron transfer (PET) process that quenches the emission through non-radiative channels. Moreover, the dynamic rotation through single bonds (in a solvated state) act in synergy with PET in consuming energy efficiently via radiationless pathways offering weak emission attributes of PHCS molecules that have not interacted. On the contrary, we rationalized a prevalence of the synergistic effect of aggregation-induced emission enhancement (AIEE) and excited state intramolecular proton transfer (ESIPT) along with the suppression of PET and intramolecular motions in the PHCS molecular system at extremely high water concentrations compared to organic solutions with low to moderate water concentrations. Non-solvent addition provides a rapid transition of the excited-state population from well-dispersed monomers to organized self-assembled nano-aggregates. This apparent reversal in the excited-state population is perspicuous with the observations of remarkable ‘level-off’ tails in the UV-vis absorption spectra, a noteworthy fluorescence enhancement (18-fold), a significant increase in the amplitude and rotational relaxation time of the longer decay components associated with aggregates, as well as the large hydrodynamic diameters of the formed spherical nano-aggregates at high water concentrations. These results inevitably suggest the opening of the excited-state radiative transitions, discernible with the observed high radiative decay rate and the simultaneous closure of the radiationless annihilations reflected from the reduced non-radiative and electron transfer decay rates. Quite expectedly, the interaction of bovine serum albumin (BSA) with the PHCS luminogen also indicated a similar ‘light up’ emission response that contrasted the amplitudes of the faster (unbound) and longer (protein-bound) photoexcited species, with a distinct fluorescence enhancement (1.4-fold). In addition, there were increased amplitude average fluorescence lifetimes (2.7-fold) and rotational relaxation times (2.1-fold) of bound fluorophores in the protein microenvironment owing to the predominant RIM effects in the hydrophobic domains of BSA. Thus, the present investigation provides a new insight into how the multifaceted photodynamics of the AIEEgen underwent striking changes in variable microenvironments, which can provide a guidance to judiciously modulate the external microenvironment to harvest optimal optical responses of small organic functional molecules.